Usually, a three sector cell-site with standard vertical polarization uses space diversity to improve the system reliability (via horizontal space diversity effective gain). This traditional approach often requires nine separate antennas. A newer approach uses three dual slant polarized antennas to give just about the same system reliability, depending on the surrounding environment, using polarization diversity effective gain. One example of this approach is the Microsite.TM. scheme, offered by Andrew Corporation, the assignee of this invention.
Polarization diversity works best for congested urban areas, but is less efficient for rural and less congested suburban areas. Typically, in less congested areas, the random polarization scattering levels or multipath encountered are relatively lower than in an urban area and often, the polarization diversity gain is minimal.
However, using space diversity usually implies 10 to 20 .lambda..sub.0 (the wavelength in free space) spacing between the two receive antennas for each sector in order to achieve meaningful diversity gain. This spacing implies a relatively large, generally triangular-cross-section support frame which can be expected to lead to generally higher windloading problems at the tower. A relatively larger support frame also can have a negative impact on cell-site aesthetics for purposes of obtaining zoning board approval, and the like. Moreover, the large number of antennas (typically nine antennas for a three-sector cell site using space diversity) implies higher windloading on the numerous antennas, as well as a further negative impact on cell-site aesthetics.
Also, the expense of providing and installing individual antenna units is multiplied by the number of antenna units required for a given multi-sector site. In this regard, operators often lease tower space for their antennas, based on the number of antenna units to be installed. Therefore, reducing the number of antenna units required for a given multi-sector coverage, reduces operator expense.
If the triangular antenna support frame can be made smaller for the same effective diversity gain, less wind loaded is expected. Correspondingly, if a fewer number of antennas can be utilized for the same effective diversity gain, less windload with respect to the antennas, better cell-site aesthetics and less overall cell-site expense can be expected.